DECLINING GROUNDWATER STORAGE EXPECTED TO AMPLIFY MOUNTAIN STREAMFLOW REDUCTIONS IN A WARMER WORLD (Invited Presentation)

Groundwater released as baseflow has the potential to buffer streamflow against short-term climate extremes, but how groundwater will respond to longer-term warming, and how this response may affect streamflow generation in mountain basins is largely unknown. Understanding groundwater flow to mountain streams is made difficult by the need to quantify the interaction of climate and snow dynamics, soil water storage and loss to vegetation, and their combined effects on groundwater recharge. Mountain basins are also comprised of complex lithologic and structural features that dictate highly heterogenous distributions of permeability, drainable storage, water table elevations and groundwater flow depths. Modeling all of these processes in mountain basins at the appropriate resolution is numerically expensive, while obtaining sufficient data to constrain integrated hydrologic model parameterization is difficult and exacerbated by challenges in site access and harsh climate. To assess the importance of groundwater on streamflow in a Colorado River headwater basin, we combined an extensive data network with an integrated hydrologic model extending 400 meters into the subsurface. Results for the water years 1987 to 2022 indicate lateral subsurface flow is a critical mechanism moving snowmelt downgradient to support streamflow and evaporative demand and to maintain groundwater levels in water-limited parts of the basin. Simulations and data indicate exceptionally dry conditions and below average monsoon rain in the last decade have initiated substantial groundwater decline. In a warmer climate, decreased snow cover and increased forest water use are predicted to reduce groundwater recharge resulting in groundwater storage loss. Losses are expected to be most severe during dry years and will likely not recover to historical levels even during simulated wet periods. Annual streamflow reductions are doubled due to groundwater depletion and intermittent streamflow is predicted in the late summer when precipitation is low. Expanding results across the region suggests groundwater declines will be highest in the HUC4 Colorado Headwater and Gunnison Basins. Our research highlights the tight coupling between vegetation and groundwater dynamics and that excluding explicit groundwater response to warming may underestimate future reductions in mountain streamflow.